An incomplete Herpes-like virus (CHV) was isolated from benign lymphoproliferations stemming from partly inbred (11-14th generation) Chinese hamsters, Cricetulus griseus (2n equals 22). The kinds of spontaneous tumors altered dramatically from solid fibrosarcomas, observed in hybrid and partly inbred animals, to lymphoreticular neoplasms and adenocarcinomas, starting at the 17-18th generations of full-sib matings. Similarly, the American hamster, Cricetulus migratorius (2n equals 22), which remained tumor-free until the 17th generation of full-sib matings, now develops only reticular cell neoplasms. The virus is lymphotropic and propagates only in sensitive cell lines maintained in glass culture vessels. CPE aborts when employing plasticware, thereby indicating dependence of the virus on some inert (ionic?) material found in glass for its replication. This study will clarify whether CHV is a latent, nonpathogenic agent having a long- standing relationship with hybrid Chinese hamsters, or if inbreeding has altered this association to one of oncogenicity. Chromosome studies on dwarf hamster and human neoplasms of B-cell (Burkitt's and non-Hodgkin's) lymphoma have revealed both karyotypes to have characteristic heteromorphic pairs of chromosomes. The single- banded nature of these intercalary duplications is unrelated to structural rearrangements, such as reciprocal translocation.